PNNL

Abstract

This study of vadose zone and aquifer sediments beneath a former dichromate spill site showed that the persistence of CrVI in the sediments and the large differences in release rate was caused by the presence of multiple CrVI surface phases. Vadose zone sediments contained numerous 1 to 10 micron high solubility calcium chromate crystals, with lesser amounts of unidentified phases indicated by Ba/Cr association in weathered pyroxenes and Ca/Cr association in weathered Ca-rich plagioclase. Most of the CrVI mass in these vadose zone sediments was readily leached in laboratory columns at very high concentrations (up to 187 mg/L CrVI) likely from the highly soluble calcium chromate that was present. Additional CrVI associated with other CrVI surface phases was additionally slowly released over 100s of hours. The source of Ca and Ba for the CrVI precipitates may be from mineral dissolution associated with the historical surface spills of CrVI as an acidic dichromate solution. In contrast, aquifer sediments contained significantly less CrVI, which was slowly released over 100s of hours. Small-sized CrVI-containing precipitates (